Argentous oxide, powder and method for making same



K. N. BROWN 3,003,935

ARGENTOUS OXIDE, POWDER AND METHOD FOR MAKING SAME Oct. 10, 1961 Filed Sept. 8. 1958 INVENTORZ KENNETH IV. BROWN BY z 1 49/ s 93s ARGENTOUS oxmn, rownnn AND METHOD non MAKING SAME Kenneth N. Brown, Teaneck, N.J., assignor, by mesne This invention relates to silver oxide and more particularly to an electrochemical method for the preparation of argentous oxide powder. I

Argentous oxide is more commonly prepared by the conventional precipitation of solutions of soluble silver salts with solutions of alkalis. More specifically, the process usually consists in the precipitation of solutions of silver nitrate b-y solutions of sodium or potassium hydroxide. While such procedures are suflicient on a small scale, they are Wholly inadequate from the standpoint of uniformity, purity and price when practiced on a scale large enough to meet the industrial requirements for silveroxide powder by such large consumers as manufacturers of silver-oxide battery electrodes or the like.

In addition, the preparation of the silver nitrate and the recovery of the nitrogen oxides and/or alkali nitrates requires major investment in capital equipment-towers, scrubbers, nitration pots etc. for the recovery and re-use of nitric acid from the reaction products. Without such recovery equipment, the cost of silver-oxide powder would be too high for advantageous use, even for special-purpose batteries.

It is an object of this invention to provide an electrochemical method for the manufacture of silver-oxide powders.

It is another object of this invention to provide highpurity silver-oxide powders that are electrochemically prepared.

It is another object of this invention to provide silveroxide powder of extremely small particle size.

It is a further and accessory object of this invention to provide a method for the manufacture of silver-oxide powder of extreme purity and within a very narrow particle-size range at minimal cost.

In its broader aspects this invention embodies the general principle of the electrochemical deposition and precipitation of silver oxide at the anode from conductive solutions in which silver oxide is insoluble and with which it does not react chemically.

According to more specific aspects, this invention embraces the method of preparing a silver-oxide powder by electro-precipitation from silver anodes in alkaline solutions containing anions which do not form insoluble silver compounds.

According to other aspects, it embraces the electro-precipitated silver-oxide powder resulting from electro-precipitation in alkaline solutions, which powder, due to its mode of precipitation, is of uniform particle size.

In addition, certain aspects of this invention include modifications of the electrochemical cell to provide for the isolation of the cathode in a special electrolyte, difierent from the electrolyte effecting the precipitation. The isolation of the cathode assures a low concentration of reduceable anions, thereby maintaining the purity of the precipitated silver oxides.

A further aspect of this invention is the provision for a porous cathode cup surrounding the cathode, at which hydrogen is liberated, in order that there be a minimum of reduction of nitrogenous salts to ammoniacal compounds (these later tend to react with silver oxide to form azides of an extremely explosive nature).

tates Patent ice The invention will be more particularly described with respect to an apparatus shown in the drawing.

The sole figure shows a plating apparatus designed for the electro-precipitation of silver-oxide powder.

The plating tank illustrated may be any suitable vessel divided into two compartments, one of which is an anode compartment 101 and the other a cathode compare ment 102, separated from each other by a porous electrolyte-permeable compartment separator The anode compartment 161 contains the silver anode 103 suspended in the electrolyte 115. The anode 103 is connected to a current-carrying bus bar 107 via anodic bus leads ms. The anode 103 should be spaced with clearance from the bottom of the compartment sufiicient to permit collection of silver oxide. Within the cathode compartment 102 is suspended at stainless-steel or other inert cathode 104 which is connected to bus bar 108 via cathodic bus leads 1G6. Each of the cathodes is surrounded by a porous electrolytepermeable cathode cup 113., the top of which protrudes above the level of the electrolyte 115 in the cathode compartment. Within each of the cathode cups 113 a second electrolyte is contained. This electrolyte is maintained at a level 116 above the electrolyte level 115' existing in the main cathode compartment.

The anode compartment may preferably be constructed with a sloping bottom to permit the accumulation and 'collection of the precipitated silver oxide; at the lowest point of the sloping bottom of this compartment a valve arrangement 112 is provided to permit the removal of the collected silver oxide. The silver oxide is then separated from the electrolyte which flushes it through the valve by means of filters, centrifuges or the like (not shown).

Referring now specifically to the cathode portion of the tank, it is preferred that the second electrolyte in the cathodecn'veloping cup should be maintained at a level at least 1 centimeter above the electrolyte level in the main tanks. As a result of this positive hydrostatic head,

- the flow of liquid through the electrolyte-permeable membrane inhibits the migration of the anions through the porous cathode cup 113 from the maintank. They are, therefore, kept away from close proximity to the reducing conditions in effect at and near the surface of the cathode. This prevents reduction of nitrates to ammonia and possible reduction of acetates.

In addition, the cathode cup 113 confines hydrogen which is liberated at the cathode to this small area and prevents the reduction and/or decomposition of nitrogenous anions present in the first electrolyte, which might be reduced if the hydrogen were not so confined.

The first or prime electrolyte 115 consists of an aqueous solution of salts of alkalis. The salts are those Whose anions form soluble silver compounds. Preferred salts of this type include sodium nitrate and sodium acetate. In general, it is preferred to maintain the salt content of the solution at that concentration which aiiords the highest conductivity. However, it has been found that the silver oxide may be prepared from electrolytes containing sodium nitrate from concentrations of 2% up to the limit of solubility.

The second electrolyte 120 may be any alkali, but sodium and potassium hydroxide are preferred due to their solubility and cheapness.

The invention will now be more particularly described in connection with the following examples:

Example I In an apparatus as shown in the drawing, with the compartments separated by a porous cloth resistant to concentrated salt solutions (Dynel) and filled with an electrolyte compound of 10% NaNO' adjusted at a pH in the 3 range 9-11 by the addition of NaOH, the silver was electrochemically precipitated from the surface of a castsilver-ingot anode. The current density ranged up to 0.15 amp./cm. The solution was not externally heated but reached an equilibrium temperature of 60 C. after V2 hour of operation. The solution was mildly stirred, but agitation was not necessary due to internal agitation promoted by the current flow.

The porous cups of ceramics or cellulosic fibers were filled with NaOH solution which was slowly added at a rate suflicient to maintain a slight hydrostatic head in the cathode cup. At low and high current densities, at temperatures ranging from that of the room up to 60 C. and at pH ranges from nearly neutral to strongly alkaline, no ammonia or ammoniacial compounds could be detected by any of the common tests. It was found that the efiiciency approached 100% at an anode current density of approximately 0.15 amp./cm.

The silver oxide powder resulting from this process, after removal from the tank, washing and drying, was found to have an average particle size of 0.7-1.2 microns as determined on Fisher Sub Sieve Sizer and yielded a powder of apparent densities in the range 0.6 to 1.0 gms./cc. using standard methods.

Example II The apparatus illustrated was charged with a 10% solution of sodium acetate adjusted to a pH range of 9-11. The other conditions described in Example I were duplicated With the exception that KOH was used for adjusting the pH and for filling the cathode cup. The silver oxide from this process was comparable in physical properties to that obtained by the process of Example I.

The argentous oxide of this invention, because of its extremely small and uniform particle size, finds an important use in the storage battery field for the preparation of special silver electrodes. Such electrodes, consisting of extremely finely powdered argentous oxide and silver, are capable of charging and discharging to only the argentous level. The voltage from such batteries is therefore extremely constant. Such electrodes have been disclosed in US. application Ser. No. 696,174, filed 13 November 1957 by Frank Solomon. Applicants electrolytic argentous oxide powder may be used directly as the positive element of silver/zinc or silver/cadmium batteries with silver powder admixed to provide conductive pathways, or it may be treated as set forth in the Solomon application.

I claim:

1. The process for electrochemically preparing substantially pure argentous oxide which comprises electrolyzing a silver anode against an inert cathode through an interposed electrolyte permeable separator in a liquid electrolyte comprising an alkaline solution of a salt having an anion which forms soluble salts with silver compounds, maintaining at said cathode a hydrostatic pressure greater than that at said anode, and collecting the silver oxides which precipitate near the anode.

2. The process according to claim 6, wherein the hydrostatic head between said first and said second electrolyte is at least 1 cm.

3. The process according to claim 1, wherein the said salt is chosen from the group consisting of alkali nitrates and acetates.

4. The process according to claim 1, wherein the elec trolyte is maintained at a pH in the range 9 to 11.

5. The process according to claim 1, wherein the electrolyzing current is approximately 0.15 amp./cm.

6. The process according to claim 1, wherein the difference in hydrostatic pressure is maintained by surrounding the inert cathode with a second electrolyte confined by another separator and comprising an alkali-metal hydroxide.

References Cited in the file of this patent UNITED STATES PATENTS 748,609 Hunt Jan. 5, 1904 1,256,758 Williams Feb. 19, 1918 2,563,729 Longsworth Aug. 7, 1951 2,773,924 Brill et al Dec. 11, 1956 2,810,686 Bodamer et a1 Oct. 22, 1957 FOREIGN PATENTS 310,099 Great Britain Apr. 25, 1929 420,402 Great Britain Nov. 30, 1934 20,192 Great Britain of 1909 

1. THE PROCESS FOR ELECTROCHEMICALLY PREPARING SUBSTANTIALLY PURE ARGENTOUS OXIDE WHICH COMPRISES ELECTROLYZING A SILVER ANODE AGAINST AN INERT CATHODE THROUGH AN INTERPOSED ELECTROLYTE PERMEABLE SEPARATOR IN A LIQUID ELECTROLYTE COMPRISING AN ALKALINE SOLUTION OF A SALT HAVING AN ANION WHICH FORMS SOLUBLE SALTS WITH SILVER COMPOUNDS, MAINTAINING AT SAID CATHODE A HYDROSTATIC PRESSURE GREATER THAN THAT AT SAID ANODE, AND COLLECTING THE SILVER OXIDES WHICH PRECIPITATE NEAR THE ANODE. 